In an airplane turbojet, it is necessary continuously to cool certain subassemblies by a flow of relatively cool air, where such subassemblies may for example be the high-pressure turbine and/or the low-pressure turbine.
By way of example, air is bled off by defining a bleed chamber in communication with a compressor, the bleed chamber having a wall that includes bleed holes to allow cooling air to escape towards the subassembly for cooling.
The wall of such a bleed chamber is generally pierced by a plurality of holes that are regularly distributed circumferentially and that define the overall flow rate of cooling air.
The flow rate is not optimized since it is necessarily adjusted to ensure sufficient cooling during periods when the turbojet is operating at high power, e.g. at full throttle as is needed during the period corresponding to takeoff. At any other speed, i.e. in particular at cruising speed or at taxiing speed, the flow rate of cooling air is much greater than that which is strictly necessary. Now, during the major portion of the time the turbojet is in use, its operating speed is specifically cruising speed.
This failure to optimize the cooling air flow rate is very expensive, in particular in terms of engine performance. The cooling air as bled, e.g. from the high-pressure compressor, does not participate in combustion. Consequently, if an excessive quantity of air is bled off, then there is a loss of efficiency and an increase in pollution.